CCD image sensors which are solid-state image sensors widely used in image pickup apparatus have been used in electronic cameras and the like in recent years. A conventional CCD image sensor, as shown in FIG. 14, for example, includes numerous photoelectric converters 191 arranged in a two-dimensional array on a plane of projection of an object image serving as an image area, CCD cells 192 and 193, output registers 194 and a charge-to-voltage converter 195. Signal charges for one frame detected through photoelectric conversion by the photoelectric converters 191 are all transferred by the CCD cells 192 to the CCD cells 193 in adjacent storage areas to be once stored therein. Then, the signal charges are transferred, in units of a predetermined number, by the CCD cells 193 to the output registers 194 arranged in a charge fetching area. Subsequently, the output registers 194 transfer the charges again, successively, to the charge-to-voltage converter 195 for conversion into voltage signals which are outputted as image signals (see Patent Document 1, for example).
(i) In recent years, two proposals have been made with regard to the CCD image sensor, to improve detection sensitivity, i.e. to realize increased sensitivity, by suppressing increase of noise and multiplying signal charges. These two proposals have a point in common in that detection sensitivity is improved by providing a charge multiplier (not shown) that multiplies signal charges by means of an impact ionization phenomenon caused by the signal charges passing through a high electric field region.
To describe the first proposal more particularly, part of the CCD cells 193 in the storage area are made to generate a high electric field region for charge multiplication. When transferring the signal charges, the charges are passed through the high electric field region for charge multiplication to multiply the signal charges, thereby to attain high sensitivity. That is, the first proposal increases sensitivity by incorporating the charge multiplier into the CCD cells 193 in the storage area (see Patent Document 2 and Non-patent Document 1, for example).
To describe the other proposal more particularly, a multiplying register is provided between the output registers 194 and charge-to-voltage converter 195 for generating a high electric field region for charge multiplication. When transferring signal charges, the charges are passed through the high electric field region for charge multiplication to multiply the signal charges, thereby to attain high sensitivity. That is, the latter proposal increases sensitivity by additionally providing a charge multiplier between the output registers 194 and charge-to-voltage converter 195 (see Patent Document 1 and Non-patent Document 2, for example).
(ii) In the case of the CCD image sensor in FIG. 14, as noted in (i), a charge multiplier (not shown) is additionally provided between the output registers 194 and charge-to-voltage converter 195. Since the signal charges outputted from the output registers 194 are converted into voltage signals after being multiplied by the charge multiplier, detection sensitivity is high for an optical image of an object. However, the speed of fetching the signal charges is not fast enough for high-speed photography exceeding 10,000 pictures per second (=10,000 frames per second).
On the other hand, high-speed image sensors capable of high-speed photography exceeding 10,000 pictures per second have already been developed. In conventional high-speed image sensors, as shown in FIGS. 15 (a) and 15 (b), each photoelectric converter 201 has a signal charge storage unit 202 for receiving, picture by picture, signal charges detected through photoelectric conversion by the photoelectric converter 201 and corresponding to a plurality of pictures, and storing the charges separately for the respective pictures. FIG. 15 (a) and FIG. 15 (b) show only part of photoelectric converters 201 arranged vertically. In reality, numerous photoelectric converters 201 are arranged both vertically and horizontally.
Each signal charge storage unit 202 includes a plurality of CCD cells 202A connected in series. As each frame is acquired during a photographic operation, a signal charge is fed from each photoelectric converter 201 to the first CCD cell 202A, and signal charges already stored at preceding acquisition steps are simultaneously passed forward by one CCD cell 202A. Thus, the CCD cells 202A of each signal charge storage unit 202 have signal charges for respective frames and corresponding in number to the CCD cells 202A, stored in the order of acquisition. The signal charges stored in the CCD cells 202A of each signal charge storage unit 202 are read after the photographic operation, converted into voltage signals, and then outputted as video signals (see Patent Document 3, for example).
Thus, in the case of the high-speed image sensors of FIG. 15 (a) and FIG. 15 (b), the photoelectric converter 201 has, attached thereto, the signal charge storage unit 202, in which the signal charges fed from the photoelectric converter 201 are transferred between the CCD cells 202A in an extremely short time, to be stored for the number of frames corresponding to the number of CCD cells 202A. Each signal charge storage unit 202 accumulates, as a lot, the signal charges for the same number of frames as the number of CCD cells 202A. It is therefore usable in high-speed photography exceeding 10,000 frames per second.
In the case of the high-speed image sensor of FIG. 15 (a), the two-dimensional array arrangement of photoelectric converters 201 does not become a square matrix. This is because, among the photoelectric converters 201 arranged vertically, lower photoelectric converters 201 are arranged obliquely leftward below upper photoelectric converters 201 so as not to overlap upper signal charge storage units 202. On the other hand, in the case of the high-speed image sensor of FIG. 15 (b), the CCD cells 202A of the signal charge storage units 202 are arranged obliquely, and lower photoelectric converters 201 are arranged directly under upper photoelectric converters 201. Consequently, the two-dimensional array arrangement of photoelectric converters 201 forms a square matrix.
[Patent Document 1]
Japanese Unexamined Patent Publication H10-304256
[Patent Document 2]
Japanese Unexamined Patent Publication H7-176721 (pages 3-7, FIGS. 1-11)
[Patent Document 3]
Japanese Unexamined Patent Publication No. 2001-345441 (page 2, FIGS. 11 and 12)
[Non-patent Document 1]
J. Hynecek, “Impactron-A New Solid State Image Intensifier, “IEEE Trans. on Elec. Dev., vol. 48, No. 10, 2001 (p. 2238-2241, FIG. 1)
[Non-patent Document 2]
M. S. Rpbbins, B. J. Hadwen, “The Noise Performance of Electron Multiplying Charge-Coupled Devices, “IEEE Trans. on Elec. Dev., vol. 50, No. 5, 2003 (p. 1227-1229, FIG. 2-3)